The incorporation of increasing numbers of discrete devices (e.g., transistors, conductive lines, conductive contact pads, etc.) into progressively smaller integrated circuits remains an important challenge in the manufacture of semiconductor devices, such as memory devices and electronic signal processors.
Many such discrete devices are fabricated using microlithography. Briefly, and in general terms, in photolithographic processes, a photolithographic mask, which is often referred to in the art as a “mask,” is formed that includes a desired pattern corresponding to a particular pattern that is to be transferred (e.g., “printed”) to a layer of material on a semiconductor die or wafer. The pattern generally includes optically transparent areas and optically opaque areas that are suitably arranged on an optically transparent supporting substrate. The mask may then be interposed between an illumination system and a layer of an illumination-sensitive photoresist material applied to a semiconductor wafer. The illumination system emits illumination radiation through the mask and onto the photoresist material. The mask allows certain regions of the photoresist material to be exposed to the illumination radiation while shielding other regions of the photoresist material from the illumination radiation, in accordance with the pattern of the mask. The exposure of certain regions of photoresist material to the illumination radiation results in changes to the properties of the photoresist material in those exposed regions. The photoresist material is then “developed,” which results in removal of either the regions exposed to the illumination radiation or the regions that were shielded from the illumination radiation. As a result, the photoresist material is provided with a pattern corresponding to that of the mask. The semiconductor die or wafer, with the patterned photoresist material then may be further processed in any number of ways to further fabricate discrete devices on or in the die or wafer.
The illumination radiation may be monochromatic. When a wavelength of the illumination radiation is greater than a minimum feature size of a pattern to be transferred to a photoresist material using a mask, various optical effects may adversely affect the quality of the resulting features formed on or in the die or wafer using the patterned photoresist material. For example, edges between transparent areas and opaque areas on a mask may contribute to diffraction of the illumination radiation, which may result in interference of the waves of illumination radiation after passing through the mask, resulting in exposure reduction in areas intended to be exposed, and exposure in areas intended to be shielded from exposure. As feature sizes in semiconductor structures decrease, diffractive effects, as well as other optical effects become more prominent limiting factors in microlithography.
Accordingly, various compensation methods are available that may increase the pattern fidelity in the structure. For example, in one known method, optical proximity correction (OPC) may be used to perturb the shapes of transmitting apertures, or other features on the mask to enhance optical resolution in the sub-wavelength regime. In general, the perturbed features on the mask are sub-resolution features since they are generally not printed onto the structure during the exposure process. Accordingly, such features are often referred to as sub-resolution assist features. Examples of sub-resolution assist features include “serifs” for reducing corner rounding in features formed in the structure, and “hammerheads” for reducing the shortening of end line features. Other sub-resolution assist features include scattering bars, or “outriggers,” and “inriggers” that improve line width control in the structure. Still other methods may be used to improve the resolution of features in the sub-resolution regime. For example, Phase Shift Masking (PSM) methods generally enable transparent areas on the mask to transmit phase-shifted illumination to the structure in order to reduce destructive interference that may occur between transparent areas that are separated by an opaque area on the mask. Still other methods may be directed to the illumination system itself. For example, an incident radiation angle (σ) and/or the numerical aperture (NA) of a projection lens may be suitably configured to resolve relatively dense lines and spaces.